The invention relates to a filling level gage operating with microwaves, for measuring a filling level of a filling material in a container, having a microwave generator and an antenna with planar antenna structure, which is used to transmit the microwaves in the direction of the filling material and to receive microwaves reflected from a filling material surface. In these filling level gages, a time of flight of the microwaves from the device to the filling material surface and back is normally established by means of a reception and evaluation circuit, and the current filling level is determined therefrom.
During the measurement of a filling level, microwaves are transmitted to the surface of a filling material by means of an antenna, and echo waves reflected from the surface are received. An echo function representing the echo amplitudes as a function of distance is formed, from which the probable useful echo and its time of flight are determined. The distance between the filling material surface and the antenna is determined from the time of flight.
The European patent application filed on Jul. 9, 1999 with the application number 99 11 7604.1 describes an antenna with planar antenna structure, which is suitable for filling-level measurement.
Such planar antennas are also described in the book xe2x80x9cEinfxc3xchrung in die Theorie und Technik planarer Mikrowellenantennen in Mikrostreifenleitungstechnikxe2x80x9d [Introduction to the theory and technology of planar microwave antennas in microstrip line technology] Gregor Gronau, Verlagsbuchhandlung Nellissen-Wolff or in the journal article xe2x80x9cImpedance of radiation slot in the ground plane of a microstrip linexe2x80x9d, IEEE Trans. Antennas Propagat., Vol AP-30, pages 922-926, May 1982.
To determine the filling level, it is possible to employ all known methods which make it possible to measure comparatively short distances by means of reflected microwaves. The best known examples are pulse radar and frequency modulation continuous wave radar (FMCW radar).
In pulse radar, periodically short microwave transmission pulses, referred to below as wave packets, are transmitted, are reflected from the filling material surface and are received again after a distance-dependent time of flight. The received signal amplitude as a function of time is the echo function. Every value of this echo function corresponds to the amplitude of an echo reflected at a certain distance from the antenna.
In the FMCW method, a continuous microwave is transmitted which is periodically linearly frequency-modulated, for example with a sawtooth function. The frequency of the received echo signal therefore exhibits a frequency difference, which depends on the time of flight of the echo signal, with respect to the instantaneous frequency which the transmission signal has at the time of reception. The frequency difference between the transmission signal and the reception signal, which can be found by mixing the two signals and evaluating the Fourier spectrum of the mixed signal, hence corresponds to the distance of the reflecting surface from the antenna. Further, the amplitudes of the spectral lines of the frequency spectrum obtained by Fourier transformation correspond to the echo amplitudes. This Fourier spectrum therefore represents the echo function in this case.
During the measurement of a filling level using only one antenna, the problem arises that meaningful measurement of the filling level is possible only if the filling level does not fall below a minimum distance from the antenna. This minimum distance, which is often referred to as the blocking distance, is due to the fact that a reception signal resulting from transmission must first have decayed to an amplitude lying below the echo amplitude before the echo signal reflected by the filling material surface can be reliably detected and evaluated.
This problem can be substantially solved by using two separate antennas, one of which is used to transmit and one to receive microwaves. But this solution requires the container to have two openings at a suitable separation, through which the two antennas can be inserted. This is, however, not the case in most applications.
EP-B 592 584 describes a filling level gage operating with microwaves, having
a microwave generator and
an antenna,
which is used to transmit the microwaves in the direction of the filling material and to receive microwaves reflected from a filling material surface,
in which a transmission element and a reception element are arranged.
Crosstalk from the transmitter to the receiver is reduced here by generating microwaves polarized linearly in a first polarization plane and passing them through a phase shifter. The phase shifter is dimensioned so that the emerging microwaves are e.g. left-circularly polarized. As a result of reflection from the filling material surface, correspondingly right-circularly polarized microwaves are then received and converted into linearly polarized microwaves by means of the phase shifter. The polarization plane of these microwaves is perpendicular to the first polarization plane. The receiver is designed so that it only receives microwaves with this polarization, but does not pick up microwaves polarized along the first polarization plane.
Such an antenna, however, is very expensive to produce since it requires corresponding filters and phase shifters. Further, it is comparatively large as a result, and power is lost every time the microwaves pass through a filter or phase shifter.
It is an object of the invention to provide a filling level gage operating with microwaves, which functions with a single antenna constructed as simply as possible, and in which a minimum distance required for measurement between the filling material and the antenna is as small as possible.
To that end, the invention consists of a filling level gage operating with microwaves, for measuring a filling level of a filling material in a container, having
a microwave generator,
an antenna with planar antenna structure,
which is used to transmit the microwaves in the direction of the filling material and to receive microwaves reflected from a filling material surface,
in which the planar antenna structure has at least two transmission and/or reception elements.
According to a preferred embodiment of the invention, the transmission and/or reception elements are respectively located in a subregion of the antenna.
According to another preferred embodiment, the transmission and/or reception elements are arranged interleaved.
According to a further embodiment, in order to measure a filling level at a close range in front of the antenna, at least one of the transmission and/or reception elements is used exclusively as a receiver.
According to still another preferred embodiment, in order to measure a filling level at a far range in front of the antenna, all the transmission and/or reception elements are used as transmitters and as receivers.
According to yet a further preferred embodiment, at least one transmission and/or reception element is used exclusively as a transmitter and the remaining transmission and/or reception elements are used exclusively as receivers, and a differential signal is established which corresponds to the difference between the transmission signals applied to the transmitters and the reception signals received by the receivers.
According to an advantageous and preferred embodiment, the microwaves to be transmitted have frequencies which are higher than 20 GHz.
One advantage of the invention is that, owing to the planar antenna structure, the antenna provides a very high degree of flexibility. The antenna structure can, as desired, be split into a plurality of transmission and/or reception elements and each transmission and/or reception element can be used optimally. By means of this, for example, a very high transmission power is available for measurements at far range and, for measurements at close range, splitting the transmission and/or reception elements into pure transmission elements and pure reception elements significantly reduces crosstalk from the transmitter to the receiver. The only additional outlay needed for utilizing these advantages involves corresponding circuit connections of the individual transmission and/or reception elements. This is simple to implement and does not entail any power reduction.
The invention and further advantages will now be described in more detail with the aid of the figures of the drawing, in which four exemplary embodiments are represented; the same parts are provided with the same reference numbers in the figures.